An apparatus of this type, designed as a muffle oven, is the subject matter of commonly owned U.S. Pat. No. 3,978,843 in the name of Wilfried Durth. As shown there, the oven has a generally closed housing of prismatic shape with rectangular horizontal and vertical cross-section. The interior of that housing is divided by a partition into a central treatment chamber and an air-circulating compartment which surrounds that chamber on three sides by forming a rear air space that opens into two lateral air spaces. A rotary blower disposed at an intermediate level in the rear air space has an intake end centered on a horizontal axis and aligned with an opening in a rear section of the partition through which air from the treatment chamber can be aspirated by the blower which is driven by an external motor. The aspirated air is expelled by the blower into the rear space of the circulating compartment from which it returns to the treatment chamber by way of the lateral air spaces and side apertures in the partition. Heating elements disposed in the circulation compartment at corners forming junctions between the rear and lateral air spaces maintain the flow at an elevated temperature designed for cooking foods resting on a rack in the treatment chamber.
The apparatus of the prior patent operates in a generally satisfactory manner but does not fully utilize the thermal energy generated by the heating elements since the air flowing past them is not uniformly distributed over their lengths. Losses of energy also occur at adjustable vanes designed to control the rate of air circulation; these losses are intensified by turbulence at other locations where the flow changes direction, as at the aforementioned junctions between the rear and lateral air spaces.
The nonuniformity of contact between the heating elements and the surrounding air flow is at least partly due to the fact that a radial blower, with a set of equispaced peripheral blades, generally emits an air stream that is not radially oriented with reference to the blower axis. Thus, the blades impart to the air stream a tangenal velocity component whose direction and magnitude are determined by the sense and the speed of rotation. This means that the air flow will be unsymmetrically divided on striking the top and the bottom of the housing; it will therefore have a greater density in the lower half of the rear air space on one side of the blower and in its upper half on the other side thereof.